1. Field of Invention
Exemplary aspects of the present invention relate to an image forming apparatus, a fixing unit, and an image forming method, and more particularly to an image forming apparatus, a fixing unit, and an image forming method using an induction heater.
2. Description of Related Art
A related art electro photographic image forming apparatus, such as a copying machine, a printer, or a facsimile machine, generally forms an electrostatic latent image on a photoconductor according to image data. The electrostatic latent image is visualized with toner to form a toner image on the photoconductor. The toner image is transferred onto a sheet and the sheet having the toner image is conveyed to a fixing unit in which heat and pressure fix the toner image on the sheet.
Such a fixing unit may use an induction heating method which can generate heat required for a fixing operation within a decreased time period so as to reduce energy consumption. Specifically, one example of a related art fixing unit includes a support roller, an auxiliary fixing roller, a fixing belt looped over the support roller and the auxiliary fixing roller, an induction heater opposing the support roller via the fixing belt, and a pressure roller pressingly opposing the auxiliary fixing roller via the fixing belt. The induction heater includes a coil which extends in a width direction of the fixing belt (i.e., in a direction perpendicular to a sheet conveyance direction) and a core opposing the coil.
A high-frequency alternating current is applied to the coil to form an alternating magnetic field around the coil. The alternating magnetic field induces an eddy current near a surface of the support roller. An electric resistance of the support roller generates Joule heat. The Joule heat is transferred from the support roller to the fixing belt while the rotating fixing belt passes under the induction heater opposing the support roller. The heated fixing belt heats the sheet having the toner image and fixes the toner image on the sheet while the sheet is conveyed through a nip formed under pressure between the pressure roller and the auxiliary fixing roller opposing to each other via the fixing belt.
In the fixing unit using the induction heating method, the support roller is directly heated by induction heating, providing an enhanced heat exchange efficiency. That is, the fixing belt can have a target fixing temperature on a surface thereof in a decreased time period with a reduced energy consumption. At the same time, however, the auxiliary fixing roller may be deformed, that is, thermally expanded due to heat transferred from the fixing belt, resulting in a change of a position of the support roller relative to the induction heater. This leads to variations of heat generating efficiencies of the support roller.